Simple and Cheap Transverter for 10 Ghz
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Simple and Cheap Transverter for 10 GHz Paul Wade, W1GHZ ©2016 [email protected] I have been working on cheap and simple microwave transverters for the past 10 years, covering all bands through 5.7 GHz. Although 10 GHz is one of the most popular microwave bands, there are still technical challenges to overcome. It has taken several attempts and some lessons learned to develop a 10 GHz transverter that I believe to be reproducible and affordable – the cost should be under $100. There are at least two good commercial transverters available, but the expense may be a barrier to those who aren’t sure they are ready for 10 GHz. The other alternative, surplus, is less available than it was when many of us got started. Design Figure 1 – Circuit side of 10 GHz Transverter The 10 GHz transverter, shown in Figure 1, looks a lot like the 5760 MHz transverter 1 – three MMIC stages for transmit and three for receive, with pipe-cap filters between stages. The differences are that everything is smaller. The pipe-caps are ½ inch rather than ¾ inch and the quarter-wave bias stubs are shorter. Most important, the PC board is thinner, 1/32 inch rather than 1/16 inch. One thing I learned while developing this transverter is that ordinary 1/16 inch PC boards radiate badly at frequencies above about 7 GHz – more about this later. The design philosophy is the same as the lower frequency Cheap and Simple Transverters 2: Gain is Cheap , provided by inexpensive MMICs. We use the cheap gain to overcome losses of the other components – ordinary chip capacitors and resistors, rather than expensive microwave parts. The PC board is ordinary FR-4 material rather than expensive Teflon boards. Ordinary copper plumbing pipe-caps provide the filters 3. The only microwave component is the mixer, and Minicircuits 4 offers surface-mount mixers at reasonable prices. Figure 2 – Pipe-cap filter side of 10 GHz Transverter I have only found one MMIC that works well at 10 GHz, the RFMD NLB-310. Consequently, all six amplifier stages are identical and use this device, as shown in the schematic, Figure 3. There will probably be other good MMICs in the future, so the PC board has footprints which will accommodate both the older 4-lead package and the newer SOT-89 package. In addition to the MMICs, there are only three resistor values and four capacitor values used. Figure 3 – 10 GHz Transverter Schematic Diagram Performance This is a basic transverter, with power output of 5 to 10 milliwatts and Noise Figure around 8 or 9 dB, enough for any line-of-sight path. For higher performance, you may add power amplifiers and low-noise amplifiers – a satellite TV LNB can be modified for a good preamp. Local oscillator power >+5 dBm is needed for best performance, but the transverter still works at lower levels. I tested it down to about +1dBm of LO injection, and output was down a few dB. LO feedthrough is down perhaps 25-30 dB, but I don’t have a spectrum analyzer to get accurate numbers. The pipe-cap filters, with two pipe-caps separated by amplifiers in each direction, provide a bandwidth of about 70 MHz with 4mm probes. Circuit details The PC board is ordinary FR-4 dielectric, probably quite lossy, but Gain is Cheap . The board is 1/32 inch thick, which seems thin enough to keep radiation tolerable – anything more than an inch away has minimal effect. With thicker 1/16” board, radiation was noticeable at 5 inches. A closeup of one of the amplifier stages is shown in Figure 4, with the components identified. All six amplifier stages are identical and use the same component identification. Figure 4 – All amplifier stages are identical The NLB-310 MMIC (Digikey 5 689-1004-1-ND) is straightforward, 50 ohms in and out, but the bias network might need some explanation. The quarter-wave stub at the top is open-circuit at the top end, so the bottom end is transformed by the quarter-wave to be a virtual ground. Then the narrow line is a high-impedance transmission line, also ¼ λ long, which transforms the virtual ground back to an open circuit. If this much works, then all the other resistors and capacitors have no effect at 10 GHz – they are there for decoupling at lower frequencies. Low frequency decoupling is important because MMICs have higher gain at lower frequencies and will readily oscillate without the decoupling capacitors to short out low-frequency paths. The radial stub has the same properties as a quarter-wave stub, but over a wider bandwidth, so the size isn’t as critical. It produces a virtual ground at the connection point to the narrow line. The narrow line is another high-impedance ¼ λ transmission line, transforming the virtual ground back to an open circuit at the 50 ohm RF line from the MMIC. Thus the bias network delivers voltage to the MMIC with almost no effect at 10 GHz – you can poke around with your finger to prove it. The only components that should be at all critical are the series blocking capacitors, C1, and the SMA connectors. I used ordinary 5 pf ceramic chip capacitors (AVX 08051A5R0XAT2A, Digikey 478-6039-1-ND). If you are so inclined, you could try real ATC microwave capacitors and see if there is any difference. The rest of the transverter is the mixer and power splitter, shown in Figure 5. The mixer is a Minicircuits MCA1-12G, which can be purchased in single quantities. The Minicircuits.com website will recommend a similar, slightly cheaper model, but the MCA1-12G has small leads extending from the package which makes it easier to verify proper soldering. Figure 5 – Detail of mixer and power splitter The power splitter is comprised of the three 33-ohm resistors below and to the right of the mixer. I used smaller 0603 size chip resistors, but larger 0805 size would probably work as well. To the upper right of the mixer is the blocking capacitor and MMIC for the transmit side, connected to the power splitter. The SMA connector in Figure 5 is the LO port. For all the lower bands I successfully use cheap Chinese SMA connectors from ebay, but I decided to stick to name brand for 10 GHz. These are by Taoglas (Digikey 931-1175-ND ) for around $3 each, but more expensive Amphenol connectors would be fine. Construction The first step in assembly is to solder the pipe caps to the board, after drilling and tapping the 4-40 screw holes in the top of the caps. Each pipe-cap location on the PC board has a hole at the center, halfway between the probe locations. I scribe lines on the bare board half the pipe cap diameter away from the center hole, making a square to line up each pipe cap. Then I put a bit of paste flux on the rim of each cap, place the caps in position, and put a ring of wire solder around the base of each cap. I solder the caps one at a time, holding the one being soldered in position with a screwdriver while heating the top of the cap with a hot air gun. The copper conducts the heat down to the board; when the joint reaches temperature, the ring of solder melts and flows around the base of the cap, without overheating the board. As soon as the solder flows, remove the heat, let it cool until the solder solidifies, then move on to the next cap. A torch could also be used, but tends to oxidize the copper, and might damage the thin PC board. A note on pipe caps. I have found three different brands at local stores and any of them should work fine. What they all have in common is that the open end is not very uniform, so sanding on a flat surface with 220 grit Wet-or-dry sandpaper is needed to make them sit level and solder cleanly. About 20% of them are so bad that too much sanding is required, so I toss them. Figure 6 – Installing probes in Pipe-cap Filter Brass screws are preferred for pipe-cap tuning, with a lockwasher. The probes are just bits of wire – I keep them straight and the desired length by using leads from an ordinary disc capacitor, as shown in Figure 6. Measure the full lead length, then insert it until the desired length is inside, and allow another 0.8 mm for the board thickness. The probe length inside the pipe cap is about 4.5 mm. Since it is hard to get the length exact, slightly longer is probably better than too short, which would be lossy. Even better than the disc capacitors are larger capacitors with the right lead spacing, like the ones in Figure 7. I found a bunch of these being used for soldering practice. The probe length can be eyeballed with a scale as shown in Figure 6, or you could make a gauge block to make them more uniform. Figure 7 – Capacitors with perfect lead spacing make probe assembly easier All the other parts are surface mount are easily soldered with a fine-tip iron and thin solder. The RF chip capacitors and resistors are the medium 0805 size, while the DC resistors and 0.1 µF bypasses can be the larger 1206 size. Referring back to Figure 4, I usually start with R2 and work around the bias components in order.